This invention relates to optical energy transmission technology and more particularly to fiber ferrules suitable for terminating optical fibers and mode stripping as may be useful with high power laser radiation transmission. The invention further relates to lensed fibers and collimators. Moreover, the invention also concerns methods of manufacturing such ferrules and lenses.
The flexibility and light weight of fiber optic cables make them well suited for delivery of high power laser beams in applications such as robotics. Since they can be readily fused in an interface to the output of high power fiber lasers, fiber optic cables are also useful for optical energy delivery, particularly beam delivery.
Optical fibers are normally constructed of at least a core, at least one cladding surrounding the core and a polymer overcoat to mechanically protect and strengthen the optical fiber. The refractive index of the core is slightly higher than the surrounding cladding such that optical radiation launched into the core is confined due to the lower index of refraction of the cladding. The higher index of the core can be uniform, a so-called “step index core” fiber, or the refractive index may vary across the cross-section, being a maximum at the center of the core in a so-called “graded index core” fiber. The protective polymer coating is purposely made with a high refractive index such that any light propagating within the cladding circumference (i.e. “cladding mode”) is dispersed into the polymer overcoat and thus removed from the fiber.
In practical usage, some fraction of light focused into a fiber will not be guided by the core. This can be due, for example, to mismatch of beam parameters into the fiber, focusing lens imperfections and dust/imperfections on optical surfaces. The component of the incident radiation that is not coupled into the core will propagate within the cladding until it diverges to the protective polymer coating where it is removed (“stripped”). If the source of laser radiation input into the fiber is a high power laser, the intensity of radiation within these cladding modes is readily capable of burning the protective polymer coating and destroying the fiber. An example is laser radiation onto industrial workpiece targets (particularly metallic targets) that is reflected with substantial power back toward the fiber such that radiation couples into the cladding circumference rather than into the core of the fiber. For this reason it is necessary to remove any cladding mode radiation at all fiber terminations before it can possibly diverge to and destroy the protective polymer coating. “Mode stripping” is the name given to the numerous techniques used to remove such cladding modes.
The purity of the fused silica fiber optic cable production process allows fiber so manufactured to contain and transmit very high power radiation. However, surface imperfections and contaminants at terminations of the fiber cause the power threshold at which damage occurs to be greatly reduced. For this reason it is preferred that the cross sectional area of the transmitted power within the fiber, the “mode size,” be increased at fiber terminations. This is commonly achieved either by expanding the core of the fiber (“thermally expanded core fiber”) or by arc fusion splicing a length of coreless “endcap” fiber of similar outside diameter to the end of the fiber optic cable at the fiber terminations. The use of an endcap allows for much larger mode sizes at the fiber termination than those achieved by expanded core techniques. A large mode at the output end of an endcap also ensures geometrically that a very low amount of reflected radiation from this surface can couple back into the fiber as feedback without the need for an angled surface—an important practical feature in many instances. An endcap is therefore a desirable feature of any high-power fiber termination.
The small diameter (e.g. 125 μm) and high flexibility of typical silica fibers require that fiber optic termination be held in a mechanically rigid structure at termination points in order to be practically useful in precise beam delivery systems. Due to the very low thermal expansion coefficient of fused silica (0.5 ppm) the choice of materials for such a mechanical structure is limited. Additionally, for example, the core of typical single mode 1060 nm wavelength compatible fiber is only 6 μm in diameter—a very small target that requires exceptional mechanical stability in order to maintain optical alignment over expected environmental conditions. Ideally such a mechanical structure would be made of fused silica itself for perfect temperature stability. Such a mechanical structure often takes the form of a ferrule—a rod-shaped part that is typically about 1 mm or more in diameter with an inner hole precisely manufactured to just accept the optical fiber within a tight tolerance. Typically optical fibers are cemented into such ferrule structures. In practice some fraction of incident laser radiation will be incident upon such adhesive bonds. When radiation from high power lasers is incident upon such ferrule end, sufficient energy may be absorbed in this bond layer to burn the adhesive and destroy the ferrule structure. Thus, a bond that does not require an adhesive is desired. To minimize transmission losses and reflected power, it is desirable to apply an antireflective coating to the end of the fiber termination. The integrity of the antireflective coating is important, and a structure is needed in which the coating is not damaged by cladding mode or mode stripped radiation. For all of the above reasons, it is desirable that any such fiber termination minimize transmission losses and reflected power, and more particularly a ferrule structure is needed that maximizes the net transmission of energy through a fiber junction.
It is often desirable to be able to produce a collimated output beam at fiber terminations. Transmitting output of a fiber through a Faraday Isolator or other component, and then focusing it back into another optical fiber is an example. In such cases, it is necessary to have well-controlled beam parameters (beam diameter and waist location) in order to achieve high fiber-to-fiber beam coupling.
The foregoing background sets forth problems of the prior art. The following patents are believed to represent a reasonable summary of prior art approaches to the some of the foregoing problems, although the invention hereinafter explained is to be recognized as a non-obvious improvement thereon.
U.S. Pat. No. 4,737,006 discloses a fiber termination composed of a silica rod arc fusion spliced to an optical fiber where a lens is formed onto the end of the silica rod with, preferably, an electric arc. The lens end is disposed adjacent to a mirror such that an optical signal transmitted down the optical fiber is reflected back into the fiber when the lens is optimally formed—terminating the electric arc heat treatment. Although such an arrangement has a desirable fused silica monolithic structure, it has no provision for mode stripping. Additionally, when applied to conventional small diameter fiber, the lens radius of curvature is necessarily very small. Both of these considerations make the previously disclosed collimator unsuitable for laser power levels in excess of a few watts.
U.S. Pat. No. 6,033,515 discloses use of a CO2 laser to fuse a fiber to a comparatively large cross section optical component (e.g., a lens). Although the patent discloses a monolithic structure, it is an inherently weak structure that requires an adhesive to strengthen the fiber to lens bond. It is also fatally flawed. It does not have any provision for mode stripping capability, laser power that does not couple into the core will be absorbed in the adhesive, ultimately destroying the device. Additionally the lens disclosed is of a fixed focal length and thickness, making the critical adjustment of beam size and waist location difficult to achieve with the desired precision.
U.S. Pat. No. 4,678,273 discloses various ways of applying a covering over the optical fiber cladding (after removal of the protective polymer coating), which effectively strip cladding modes into further outer absorptive layers. Alternate structures are as effective. For example, optical fiber constructed with large diameter cladding would achieve many of the objectives of this invention. The patent does not disclose any monolithic mechanical structure and does not include an endcap. Absorption of cladding modes in the absorptive layers discussed would tend to heat the device, making mechanical alignment stability problematic. If water were to cool the portions of the device that have exposed cladding only, it would be would be problematic. Water is known to spontaneously cleave bare fiber through attack of defect sites (“Griffiths micro-cracks”)—especially if the fiber is stressed in any way. All of the above render the disclosed invention incomplete and unsuitable for use with very high power laser beams.
U.S. Pat. No. 4,575,181 discloses removal of the polymer cladding in a region near the termination of an optical fiber exposing the cladding layer. The surface of the exposed cladding is formed into a rough surface which effectively mode strips cladding modes onto a mechanical holder. The holder supports and contains a suitable length of the end of the fiber in relation to a focus lens mounted to one end of the holder. This patent does not disclose endcaps and is not a monolithic structure. Any thermal expansion mismatch would contribute to instability in the optical alignment of the structure—especially when the mechanical holder absorbs high power. Etching the fiber to cause cladding modes to be stripped as disclosed renders it susceptible to cleaving, especially if it is mechanically stressed in any way. The mechanical housing makes the device bulky in practice, and difficult and expensive to manufacture.
U.S. Pat. Nos. 6,752,537 and 6,883,975 disclose a fused silica ferrule bound to an optical fiber via a layer between the two and having a softening temperature lower than the ferrule and fiber, or having a greater absorption of radiation than the ferrule and fiber. This creates a sealed region between the ferrule and fiber, but it is not of strictly monolithic composition because the disclosed layer needs to be doped heavily enough relative to the fiber and ferrule to substantially change its thermal properties. Importantly, they also provide no means for mode stripping.
U.S. Pat. No. 5,619,602 discloses a silica rod of larger diameter than the optical fiber fused to the end of the fiber. This rod is intended to direct rays entering outside the fiber onto an annular reflector which surrounds the fiber, the reflector deflecting the rays onto an absorbing, heat sinking metallic housing surrounding the assembly. A temperature sensor on the reflector protects the assembly from damaging levels of radiation. A capillary tube in optical contact with the fiber cladding also strips cladding modes between the silica rod and the annular reflector onto the housing inner wall. This invention appears difficult to practice with small diameter fiber due to difficult and fragile assembly and evident susceptibility to vibration modes of the mode stripper capillary. Although the invention can, with detailed engineering, perform all of the desired optical fiber termination functions, it is realized in a long, bulky and expensive device, rendering it impractical in many applications.
U.S. Pat. No. 6,167,177 discloses water cooling of an optical fiber termination. A bare end of optical fiber is in optical contact with a transmissive window. The window and fiber are surrounded by a sealed, absorbing housing. In one embodiment, water flows in the space between the optical fiber and the housing. Laser radiation is coupled into the fiber through the window. Radiation that does not couple into the fiber in any way is absorbed in the water and at the water cooled housing surface. Cladding modes can be stripped by using an additional optically contacted capillary tube that is suspended within the structure by the fiber itself or by roughening the cladding surface of the fiber by diamond grinding or chemical etching means. It may be observed that either method of mode stripping is problematic with regard to reliability. Both mode stripping means have bare fiber in contact with water, potentially allowing spontaneous cleavage at defect sites, especially for the disclosed roughened surface approach. Additionally suspending the capillary mode stripper by the fiber itself does not appear robust with respect to shock and vibration modes—at least for small diameter fibers. An alternative embodiment surrounds the fiber with a transparent tube sealing and protecting the fiber from contact with the cooling water. However, since an air gap exists between the transparent tube and the fiber no mode stripping function is performed.
It is thus seen that all known prior art techniques, devices and inventions have specific and general disadvantages for which solutions have not been suggested by the prior art.